Cloud streets

are bands or lines of
cumulus cloud that are oriented almost parallel to the low-level wind direction (with a slight angle of about 10 to 20 degrees to the left).

They usually form within the lower one to three kilometers of the atmosphere known as the planetary boundary layer and are caused by
convection when the wind direction is relatively constant with height. Multiple sources of thermals and a constant wind speed often give rise to a series of longitudinal roll vortices visible as cloud streets. This effect may be accentuated when a temperature inversion is capping the convection. The space between the cloud streets (the non-cloud streets) is about two or three times the depth of the convective layer.

Usually cloud streets have a well defined start point often following coast lines, but the end points tend to tail off as the air moves away from the source of convection. Cloud streets may from over seas as relatively cool air streams over relatively warm water, or they may be form over land as relatively unstable air streams over an obstacle and convection is caused by friction or shear. The latter can be seen on a
satellite picture of Wales, taken around mid-day on March 5th, 2002. The cloud most frequently appearing in cloud streets is Cumulus mediocris.